Project Summary/Abstract The ability of cells to robustly enter and exit quiescence is essential for tissue homeostasis, and defects in the transition between proliferation and quiescence lead to diseases of excess proliferation (cancer) or cell loss (aging and degeneration). Critical to this transition are gene expression changes regulated by well understood signaling pathways. In addition to transcriptional changes, chromatin modification and accessibility is regulated between quiescent and proliferating cells, and the functions of chromatin modifiers have been shown to disrupt these transitions. However, the mechanisms by which chromatin affects the ability of cells to transition between quiescence and proliferation are unknown. The goal of this proposal is to characterize global and gene-specific chromatin modification and accessibility changes, and define how chromatin regulation affects the ability of cells to transition in and out of quiescence. The central hypothesis is that distinct chromatin modifications regulate the entry, exit, and maintenance of quiescence through control of both chromatin accessibility and site-specific transcriptional control of critical cell cycle genes. This hypothesis will be tested in three specific aims: (1) Define the functional role of global changes in chromatin modifications in proliferating and quiescent cells. (2) Identify and induce site-specific chromatin modification changes at the promoters of critical cell cycle regulated genes. (3) Determine changes in chromatin accessibility in quiescent and proliferating cells. Global and site-specific chromatin modifications changes will be identified in quiescent and proliferating human epithelial cells. The role of these changes in cell cycle regulation will be determined by disrupting the chromatin regulators responsible for the deposition and removal of the modifications of interest, and using time lapse microscopy to image live-cell sensors for activities critical to cell cycle progression. This quantitative method will identify aspects of the cell cycle, which are perturbed by changes in chromatin modification, including probability of quiescence and time to cell cycle re-entry. Moreover, changes in chromatin accessibility in this critical transition will be identified, and the role of modifications in regulating chromatin accessibility will be evaluated. This work will contribute to the field by defining a novel pathway, chromatin dynamics, which functions to regulate cell cycle entry and exit. Furthermore, the proposed work will advance the understanding of the mechanisms by which chromatin modifiers control the establishment and maintenance of chromatin marks. Finally, the proposed work will offer new targets for therapeutic treatment of diseases of tissue homeostasis, including cancer.